1. Field of the Invention
The present general inventive concept relates to power factor correction, and more particularly, to a power factor correction method and apparatus adapted for used in a power supply which provides a direct-current (DC) power using a capacitive input type smoothing circuit, and an improved power supply using the same.
2. Description of the Related Art
Recently, power supplies which can be stable, compact and light have been developed to be applied to factory automation devices, office automation devices, communications devices, and power systems. In addition, power factor correction and harmonics reduction become important issues in developing the power supply.
For example, European Union forcibly restricts use of current of harmonics causing an electromagnetic interference in order to enhance quality of power supplies and heighten a trade barrier to protect products made in the European countries. Japan also restricts a general voltage distortion ratio of a power system via its suppression guide line based on the IEC standards, in order to maintain a harmonics environmental target level, in which all devices generating harmonics are regulated.
Accordingly, devices which generate current of harmonics more than a restricted value recommended by an importing country cannot be exported to the importing country. In Korea, the radio wave law amended in 1992 stipulates that a person who manufactures or imports electromagnetic interference (EMI) devices is required to obtain an official approval of EMI with respect to the devices.
In general, a power supply includes a rectifier which converts an AC voltage into a DC voltage, and a DC-DC converter which stabilizes an output from the rectifier in correspondence to variation of loads and an input voltage. A capacitive input type rectifier which is widely used as a DC power supply for various electronic equipment needs a capacitor having a large amount of capacity to correspond to an instantaneous power failure or reduce a burden of the DC-DC converter by suppressing variation of an output voltage.
However, as the capacity of the capacitor becomes larger, a pulse-shaped large current is required to flow in order to store a large amount of energy in the capacitor in a short period of time. In this case, a peak value becomes five to ten times as many as an effective value. A waveform of an input current of the rectifier becomes discontinuous due to the pulse-shaped large current. The pulse-shaped large current influences EMI upon peripheral devices due to distortion of the input voltage and a harmonics component of the input current.
Many efforts have been made on a method of adding a power factor correction circuit (PFC) to a DC-DC converter in a switching power supply.
A conventional power factor correction circuit for use in a power supply is divided into a passive PFC and an active PFC.
FIG. 1 is a block diagram showing a power supply to which a conventional passive power factor correction circuit is applied. In FIG. 1, an inductor L is interposed between a rectifier 102 and a smoothing capacitor C or a DC-DC converter 130 to widen a conducting angle of a charging current of a capacitor C using impedance of the inductor L, thereby enhancing power factor correction.
FIGS. 2A and 2B are graphs schematically showing a waveform of operations in the apparatus shown in FIG. 1. FIG. 2A illustrates a charging current Ic in a case that there is no inductor, and FIG. 2B illustrates a charging current Ic′ in a case that there is an inductor.
In the case that there is no inductor, the charging current Ic flows only when a voltage Vr applied to the smoothing capacitor C exceeds a charging voltage Vac of the smoothing capacitor C. As shown in FIG. 2A, a pulse-shaped large current flows only near a peak value of an input voltage, that is, near phases of π/2 to 3π/2 of the input voltage. A power factor is lowered and, many kinds of current of harmonics are induced by the pulse-shaped large current.
Such a passive power factor correction circuit is widely used in a field of handling a low-band frequency, a low EMI, and a high power.
FIG. 3 is a block diagram showing a power supply to which a conventional active power factor correction circuit is applied. In the power supply shown in FIG. 3, a waveform of an input current rectified by a rectifier 302 is changed to be similar to a sinusoidal waveform by using a semiconductor switch Q2, thereby making a phase of the rectified input current equal to a phase of an input voltage. A two-stage power factor correction circuit is chiefly used as an active power factor correction circuit. Here, a rectifying portion of an existing switching power supply is replaced by a power factor correction circuit 304 as a pre-regulator, and DC-DC conversion is performed by using a post-stage regulator 306. That is, since the active power factor correction circuit maintains a high power factor within a broad input voltage range, the active power function correction circuit is appropriate for devices which form a multiple output, require insulation, and need high precision.
The conventional power factor correction devices shown in FIGS. 1 and 3, use an inductor L to enhance power factor correction. Thus, such an inductor L used in the power supplies should be an inductor having a large inductance to enhance the power factor correction since the inductor should be designed to correspond to a frequency of an input AC voltage, such as a frequency of a commercial AC voltage which is 50 Hz or 60 Hz. Accordingly, the inductor used in a power factor correction device becomes large in shape and weight and high in price. Also, the power factor correction devices which use the inductor L has a power factor correction effect which is greatly influenced by variation of an input voltage and current and variation of an output voltage and current, and a very large power is consumed in a linear regulator corresponding to a post stage of the power factor correction stage.